1. Field of the Invention
The present invention relates to a semiconductor laser module to guide laser beams emitted from a semiconductor laser to an end face of an optical fiber core, particularly to a coupling structure of a semiconductor laser and an optical fiber in the semiconductor laser module in which the end face of the optical fiber core is cut slant in order to prevent the laser beams from returning to the laser.
2. Description of the Related Art
Generally, this type of a semiconductor laser module is provided with a construction as shown in FIG. 1, in which a semiconductor laser 1 emits laser beams, and a coupling lens 2 converges the emitted laser beams so as to fall on an end face of a core 4 of an optical fiber 3. Further, the end face of the core 4 is cut slant so as to prevent the laser beams reflected on the end face from returning to the semiconductor laser 1.
This type of coupling structure is disclosed, for example, in the Japanese Patent Publication No. Hei 4-66324. According to this, the structure is designed such that the laser beams fallen on the end face 5 of the core 4 cut obliquely by an angle .phi. from a surface perpendicular to the optical axis of the optical fiber 3 penetrate in the direction parallel to the optical axis of the core 4. To achieve the foregoing, an angle .theta..sub.2 made by an optical axis 6 of the outgoing beams from the coupling lens 2 and an optical axis 7 of the optical fiber 3 needs to satisfy the following equation, based on the Snell's law, EQU .theta..sub.2 =sin.sup.-1 (n.sub.1 sin .phi.)-.phi. (1)
here, n.sub.1 : refractive index of the core 4.
The coupling structure of the foregoing disclosure is made such that the outgoing point of beams from the semiconductor laser 1 is dislocated by a distance r.sub.1 from the optical axis of the coupling lens 2. Here, if the focal length of the coupling lens 2 is f, according to the equation (1), the distance r.sub.1 is given as follows. EQU r.sub.1 =f sin .theta..sub.2 (2)
However, generally deviating the position of the optical axis of a light source from the optical axis of a lens will increase an aberration virtually in proportion to the deviation r.sub.1, as shown in FIG. 2. The foregoing conventional example only considers the Snell's law shown in the equation (1) and (2), and does not consider this aberration; and therefore, the example is worse in the coupling efficiency, which is a problem. Further, the coupling efficiency becomes asymmetric, depending on a fitting error deviated upward and/or downward from the position of the semiconductor laser 1 being dislocated by the distance r.sub.1 from the optical axis. Therefore, the dispersion of the coupling efficiency becomes large, which is a problem.